The present invention relates to a smoke-permeable non-reinforced film based on regenerated cellulose, which has a high tear strength in the longitudinal and transverse directions. The invention also relates to a tubular casing comprising the film and to a process for the production of the film, in which a web-shaped film of viscose is coagulated, regenerated to form cellulose hydrate gel and then dried and thus converted into regenerated cellulose. The film is used for manufacturing the tubular sausage casings by forming a tubing from the web-shaped film and joining it along its edge zones. Additionally, the invention relates to a skinless sausage product which has been formed by a tubular casing comprising the present film.
It has been known for a long time to produce web-shaped, non-reinforced films of regenerated cellulose (cellophane, cellulose hydrate), according to the viscose process, e.g., see U.S. Pat. No. 2,991,510. These films are used for various packaging applications, in particular as wrapping films.
The manufacture of sausage casings from such flat film webs is described in some recent publications, e.g., European Patent Applications No. 0,037,543 and No. 0,058,240. For this purpose, the web is bent to form a tubing and bonded, either along its overlapping edge zones or by means of a film strip which covers the two edge zones. It has now been found, however, that the conventional non-reinforced regenerated cellulose film, i.e. a film without fiber-reinforcement, which is conventionally employed as packaging films, is not suitable for use in the manufacture of a sausage casing having a glued seam, since this film can hardly withstand the mechanical stresses involved in the processing of a sausage casing. In the mechanical production of sausages which are to be smoked, particularly sausages of the cooked-sausage type, the casing material must meet considerably higher mechanical requirements than in the case of other types of packagings. In the stuffing, smoking, scalding and drying procedures, the casing material is stretched to a considerable extent and during portioning, i.e. linking of individual sausages, the material is strongly twisted. Moreover, high-speed mechanical shirring of the tubings into shirred sticks and also the usual mechanical peeling of the tubular casing from the sausage meat to produce skinless cooked sausages (frankfurters or wieners), subject the casing material to high mechanical stresses which the conventional regenerated cellulose films cannot withstand.
As disclosed in European Patent Application No. 0,076,436, a web material of cellulose without fiber-reinforcement which can be employed in manufacturing sausage casings having a glued seam, should have a tear strength in the wet state, which ranges from about 15 to 30 N/mm.sup.2 (DIN 53 455), both in the transverse and the longitudinal directions, with the tear strength in the transverse direction being equal to or higher than the tear strength in the longitudinal direction. The resulting degree of orientation, i.e., the quotient of tear strength in the longitudinal direction and tear strength in the transverse direction, in each case measured according to DIN 53 455, should have a value of about 1, and preferably, less than 1. According to European Patent Application No. 0,076,436 a film is produced which meets these criteria by conventional longitudinal and/or transverse stretching.
A non-reinforced film of regenerated cellulose having a comparatively high tear strength has the advantage of an improved resistance to certain mechanical stresses, such as those which occur when sausage meat is pressed into the sausage casing and when the sausage is scalded. It has become apparent, however, that high tear strengths alone do not render the conventional films of regenerated cellulose suitable for manufacturing sausage casings having a longitudinal seam, which are to be portioned to give individual sausages, by twisting the casings in the process of stuffing them with sausage meat. Portioning is carried out in such a way that the pressure-loaded, filled section of the tubular casing is repeatedly twisted at particular intervals, corresponding to the desired length of the sausages. In the process, the casing material which is under tension and is moistened throughout, is locally highly stressed, with respect to its splitting resistance and its twisting capability. In addition, the sausage meat must be displaced and pushed away from the part of the casing, which is to be twisted. The conventional cellulose films produced in the form of webs cannot withstand these stresses due to brittleness of the films.